L-alanine, also termed (S)-2-aminopropionic acid is a proteinogenic amino acid, which may be may be utilized in clinical nutrition or as an ingredient for pharmaceutical products such as therapeutics for prostate hypertrophy. It is also used in the food industry, to enrich nutrition in health foods and drinks. Furthermore L-alanine may be used as a supplement for cell culture. Apart from that, L-alanine may be utilized as a starting material in the production of other chemicals, such as methyl glycide diacetic acid. L-Alanine is frequently manufactured using enzymatic catalysis or fermentation processes (see e.g. Xueli Zhang et al., Production of L-alanine by metabolically engineered Escherichia coli, Appl. Microbiol. Biotechnol. (2007) 77:355-366 and the literature cited therein; Ullmann's Fine Chemicals, Wiley-VCH, Weinheim 2013, Vol. 1, pages 179 f. and the literature cited therein).
In the manufacture processes, L-alanine is usually obtained as an aqueous solution, from which it is may be isolated by spray drying or crystallization. Spray drying of an aqueous L-alanine containing solution, however, initially results in an amorphous solid powder having unsatisfactory flow characteristics and poor storage properties, as the powder may undergo caking and turn into solid lumps, which are difficult to handle.
When trying to isolate L-alanine from the aqueous solution by crystallization, one faces several problems. One problem is the tendency of L-alanine to form supersaturated aqueous solutions, which spontaneously may crystallize to form an impure solid material. A second problem is that upon crystallization of L-alanine from an aqueous solution a needle or rod shaped material is obtained, which is difficult to filter. Due to its high aspect ratio and the resulting high specific surface, such a needle or rod-shaped material contains large amounts of mother liquor and therefore it is only of moderate purity and requires additional drying effort. The formation of a needle or rod-shaped material is particularly problematic, if the aqueous solution of L-alanine stems from a fermentative production process.
The inventors of the present invention found that the formation of needle or rod shaped crystals may be ascribed to the presence of organic impurities, in particular α-amino acids different from L-alanine, such as D-alanine and certain L-amino acids, e.g. L-valine, L-leucine, L-lysine, L-asparagine, L-glutamine and L-arginine, especially to the presence of L-valine. The inventors have found that the detrimental effect of these α-amino acids on the crystallization of L-alanine may cause such problems even at very low concentrations of 100 ppm, based on the weight of L-alanine, or lower. On the other hand, the production processes of L-alanine usually contain α-amino acids different from L-alanine in significant amounts, e.g. in amounts of 1000 ppm, based on L-alanine, or higher. These impurities may result from a racemization or may be formed as byproducts in the production process.